1. Field of the Disclosure
The disclosure herein relates generally to apparatus and methods for formation testing.
2. Description of the Related Art
Oil wells (also referred to as “wellbores” or “boreholes”) are drilled at selected locations in subsurface formations to produce hydrocarbons (oil and gas). Well tests in which pressure of the well is recorded over a time period are performed to estimate well or reservoir properties, determine the productivity of the well or obtain reservoir management data. A well test referred to in the industry as “drill-stem” test is an example of such a well test. In a typical drill-stem test, a driller isolates a region or section of the wellbore. The flow volume of the formation fluid is measured. A valve and a pressure transducer are lowered down a drill pipe in the wellbore. A packer is expanded to isolate a region of the wellbore. The valve is then opened, which causes the pressure at the wall of the wellbore to fall sharply and allows the formation fluid to flow into the wellbore. Such a state is generally referred to as the “flow” state. During the flow-state, the pressure decreases over time. The variations in the pressure are recorded. Also, the volume of fluid flowing in the well is recorded. The valve is then shut for a time period (referred to as the “shut-in” period), causing the pressure to build up on the wall of the wellbore. The rate of recovery of the pressure is recorded during the shut-in period. The rate of recovery of the pressure, combined with the known amount of fluid produced during the test enables an operator to estimate properties of the formation, such as permeability and far field pressure.
Such a drill-stem tests is performed over a long time period. In some situations, however, it is desirable to perform short drill-stem tests at different wellbore depths to estimate the various formation properties. A dual packer module on a wireline is often used to perform the functions performed during a drill-stem test, but on a smaller scale. Such tests are referred to as mini drill-stem tests. A mini drill stem-test investigates a smaller volume of formation fluid due to smaller isolated region (for example three feet versus tens of feet) and withdraws a smaller amount of fluid at a lower flow rate. When performing a mini drill-stem test, it is desirable to generate a sufficiently large pressure drop in order to maximize the depth of investigation of the permeability measurement as well as to increase the signal to noise ratio of the pressure build up. Therefore, a large volume drawdown chamber is used in combination with a flow control mechanism to generate a sufficiently large pressure drop while maintaining a near constant fluid flow rate. A variable pressure control draw down chamber is often used to allow for a controlled pressure drop while measuring the fluid flow rate into the draw down chamber in real time. Such tests, however, do not allow for performing drill-stem tests at various depths during a single trip into a well. Therefore, it is desirable to provide an apparatus for performing mini drill-stem tests at multiple depths during a single trip in the wellbore.